Induction motor speed control



Feb. 17, 1959 A. B. NEWTON INDUCTION MOTOR SPEED CONTROL 2 Sheets-Sheet1 /N VENT 0/? ATTORNEYS.

Feb. 17, 1959 A. B. NEWTON 2,874,345

INDUCTION MOTOR SPEED CONTROL Filed June 24, 1957 2 Sheets-Sheet 2 TORQUE I /NVENTOR.-

wmim, ydhwifin ATTORNEYS.

2,ii7d,345

INDUQEHQN MGTGR SPEED OGNTR U L Alwin E. Newton, Wichita, Kano assi norto The Coleman Company, Inc, Wichita, Kane, a corporation of KansasApplication Jane 24, W57, Serial No. 667,553 11 Claims. (#Cl. 313-232.)

This invention relates to alternating current motors, and especially toinduction motors. More particularly, the invention is concerned withcontrolling the speed of induction motors whereby a relatively constantoperating speed is provided irrespective of voltage disturbances in thesupply lines therefor or in variations of torque requirements imposedupon the motor by its load within prescribed limits.

The use of direct drive blowers in air conditioning and heatingequipment has increased the importance of small electric motors inrespect to control of the speed thereof, and also with respect to thevariability or the speed thereof. Today, speed control is accomplishedin such blowers by using permanent split capacitor motors or shaded polemotors, since such motors are voltage-sensitive. An adustrnent thereofto various speed requirements may be accomplished by varying the voltagesupplied thereto by means of a reactor or an auto-transformer. it isapparent that since the speed of such motors can be controlled bychanging the value of the voltage supply therefor, these motors are ofnecessity voltage-sensitive and any disturbances or changes in the valueof the voltage supply will have an immediate effect on the speeds atwhich the blowers operate. This, of course, is a disadvantage.

Conceivably, such results could be avoided by using a voltage sensitivemotor equipped with a governor functional to open and close the motorsupply circuit as the speed of the motor begins to vary from apredetermined value. This would tend to stabilize the motor .9 so, butbecause the contacts that would be opened and closed by the governorwould have high voltage thereacross, considerable interference would beexperienced in closely located electrical appliances (such as radio andtelevision receivers); and further, the high voltage would cause rapiddeterioration of the contacts. it would be appreciated also that asubstantial problem is presented of providing an acceptable means fortransferring the motion from a rotating element to a fixed set ofcontacts. Therefore, such a system also has considerable inherentdisadvantages.

An object of this invention is to provide an improved for controllingthe speed or" an alternating current motor whereby the disadvantagesinherent in other speed control systems are overcome. Another object or"the invention is to provide a system for controlling the speed of aninduction motor by selectively changing the resistance of the rotorthereof to make the motor selectively voltage-sensitive andnon-voltage-sensitive or, stated another way, to provide a system forcontrolling the speed of an in.=uction motor wherein for any givenvoltage, the motor is alternately made more or less sensitive to torquevariations with respect to speed.

Still another object is to provide a system in combination with aninduction motor whereby at some predetermined speed the resistance ofthe rotor is changed preferably by a governor, the resistance beingdecreased when Patented Feb. 17, 1959 the motor speed falls below suchpredetermined speed, whereby the motor is less voltage-sensitive, andvice versa. A further object of the invention is to provide an inductionmotor wherein the rotor has a winding cast therein providing conductorbars and a slotted conductor ring shorting the conductor bars and inwhich a governor mechanism is carried by the rotor and is operative toselectively short across the slots in the shorting ring whenever thespeed drops below a predeterr ed value, thus giving a low resistance tothe rotor, but which removes the shorts across the slots in the couplingring whenever the motor speed reaches or exceeds a predetermined valuethereby restoring full resistance to the rotor an causing it to slowdown somewhat if the load on the motor is not changed. This altrnateaction of shorting the rotor and restoring its normal resistance willoccur at a speed change dependent upon the sensitivity of the governor,iereby permitting ad; stment for any desired operating speed and for anydesired sensitivity. A rtional objects and advantages will appear as thespecification develops.

Embodiments of the in ention are illustrated in the accompanying drawins, in which l is an on view in elevation of an irlmctance motor rotorembed; invention- Pig. is a partial vertical sectional view of the otor;3 is irc t view in elevation of a rotor embodying a Iorm of theinvention; Fig. a rront view in elevation of a further A0ditication;Fig. 5 is a broken transverse sectional view of the modification shownin 4, an be" taken along the line 55 thereof; 6 is a at viev inelevation oi an induction motor rotor showing still another modifiedform of the invention; 7 is a broken longitucnal sectional view of theform or" structur shown in 6' 1 is a graph wherein mote tor speed. Therotor illustrated in Figs. 1 and 2 is designated generally with thenumeral FF, and as has been brought out hereinbefore is adapted for usewith an induction motor, and for simplicity the stator of such a motoris not illustrated. The rotor is equipped with a shaft 11 that defines arotational axis thereof, and that shaft, as is well understood, will berotatably supported in bearings provided by the stator assembly and whenthe indnction motor is assembled. in accordance with conventionalpractices in the manufacture of inductance not-or rotors, the oneillustrated in Figs. 1 and 2 has a winding cast therein, and thiswinding has a plurality of bars or conductors 12 suitably separated fromeach other by laminations 12a.

it is to be appreciated that conventionally a larger number of rotorbars or conductors 12 are provided than is shown in Fig. 2. The rotorbars or conductors 12 are electrically coupled by a continuous shortingring 13 (best seen in the lower half of Fig. 2). in the illustrationgiven shorting rin i3 is provided with a plurality of slots 13a whichextend partway therethrough, providing portions of less cross-sectionalarea, and greater resistance. The rotor illustrated is conventional inthat it possesses continuous ring-shaped shorting rings ordinarilyprovided in induction motors to terminate and connect the rotor bars.Preferably, in too illustration given, the slots 1.3a are cut partwaythrough ring 13 at points between the bars 12. passing through therotor. As a result high resistance to current flow through rings 13occurs in the area of the slots whenever they are not shorted out byother means.

The rotor may be considered a high resistance rotor for the resistancethereof is purposely established at a value above what would be employedif a constant speed were desired for the induction motor using thisrotor.

asvgsae The reasons for this will be brought out in greater detailhereinafter, but from what has been set forth about it will beunderstood that the speed for this rotor is not constant and theresistance thereof is selectively varied between high and low values.

Rigidly secured to the shaft 11 by a set screw, a press fit or othersuitable means, is a mounting or support member 14 having a laterallyextending flange 15. In facing relation with the flange 15 is a governoradjusting plate 16 having an enlarged central opening 17 that spacesthat same from the shaft 11, and an arcuate slot 18 spaced laterally ofthe central opening. The plate 16 is adjustably secured to the flange 15by means of a cap screw 19 that extends freely through the elongatedarcuate slot 18 and is threadedly received within the flange 15. Theplate 16 is pivotal with respect to the shaft 11 and flange 15 withinthe limits defined by the arcuate slot 18 and the cap screw 19.

As is seen most clearly in Fig. l, the plate 16 is provided with aplurality of spaced apart cam slots 20 that are angularly oriented withrespect to radii of the annular plate 16 and of the shaft 11 which iscoaxial therewith. in being so oriented, each slot 20 has one end thatlies in close proximity with the outer edge of the plate 16, and anotherend that is inwardly thereof. There is a slot 20 for each of thechannels or spaces 13a.

Slidably carried in each of the slots 20 is a laterally turned end 21 ofpin 22 that extends outwardly of the plate 16 through a slot 13a in ring13. The ends 21 are anchored within the slots by means of clamp collarsor washers 23 and 24 on opposite sides of the plate 16, and of which atleast the washer 24 is secured to the end 21. Slidably mounted on eachof the rods or pins 22 for longitudinal movement thereon, is a shortingbar 25 normally biased into engagement with the ring 13 on each side ofthe slots 13a by means of a coil spring 26 that seats thereagainst atone end, and that at its opposite end seats on a washer or stop member27 secured to the pin 22 adjacent the outer end thereof.

The shorting members 25 are preferably formed from a good conductingmaterial, and are adapted to establish a good electrical contact betweenportions of ring 13 so as to effectively short across the slots 13a. Asthe rotor rotates about the longitudinal axis of the shaft 11,centrifugal force developed by the shorting bars 25 tends to move thosebars outwardly along the pins 22. It is clear that at some predeterminedspeed, the centrifugal force will be of sufiicient magnitude so as toovercome the biasing force of the springs 26, and at such time theshorting members 25 will move away. from the ring 13 and in interruptingthe electrical connection therebetween will no longer short across theslots 13a. When the speed of the rotor falls below such predeterminedvalue, the biasing force of the springs 26 will overcome the centrifugalforce and will cause the shorting members 25 to again short over theslots 13a.

It is to be appreciated that the shorting bars 25 are in reality lowresistance shunts for the relatively high resistance portion 13b of ring13. Portion 13b is the small cross-sectional area portion in ring 13provided by slots 13a. It is also to be appreciated that slots 13a couldbe extended completely through ring 13. In this instance when shortingmembers 25 are disengaged from ring 13, only eddy currents would producetorque in the rotor bars which would then be completely isolatedfrom'each other. The changes in current in such a case might be quitedrastic and cause upsets in the transmission line feeding the motor.

The particular speed at which the shorting elements '25 move away fromring 13 will depend upon the strength of the springs 26. Further, thebiasing force exerted by the springs may be varied by rotating the plate16 with respect to the shaft 11 and flange 15. Such rotation isaccomplished by loosening the cap screw 19 and again tightening it afterthe plate has been rotated. As viewed in Fig. l, rotation of the plate16 in a counter-clockwise direction with respect to the rotor and theshaft 11 thereof will increase the biasing force of the springs 26 bydrawing the pins 22 inwardly toward the shaft 11. On the other hand,rotation of the plate in a clockwise direction will decrease the biasingforce of the springs in that it permits the pins to move outwardly Whichallows for expansion of the springs.

The rotor embodying the modification illustrated in Fig. 3 is designatedgenerally with the numeral 30, and is supported for rotation on a shaft31. The rotor has cast therein a plurality of bars electricallyseparated from each other by gaps as specified above and coupledtogether by ring 32. The gaps or spaces 33 in ring 32 are adapted to beshorted across by internal shoes 34 and 35 supported interiorly of thebars 32 by ears 36 and 37, respectively, provided with elongated slots33 and 39 extending along a diameter of the rotor that slidably receivepins 40 and 41, respectively, that permit the shoes to move along adiameter of the rotor, but which prevent displacement of the shoes inother directions. The pins 40 and 41 preferably have enlarged heads asshown, and the slots 38 and 39 have enlargements at their outer ends topermit insertion of the enlarged heads therethrough in mounting theshoes on the pins.

Rigidly secured to the shaft 31 to prevent relative rotationtherebetween is a mounting plate 42 having stationary pins 43 and 44extending forwardly therefrom along a diameter of the rotor displacedslightly from the diameter along which the pins 4! and 41 extend.Pivotally supported on the pins 43 and 44 are governor plates or leaves45 and 46 equipped, respectively, at their outer ends with weights 47and 48 displaced from the pivotal axes 43 and 44.

The leaves 45 and 46 at the ends thereof opposite the weights arepivotally connected, respectively, with the shoes 34 and 35 by means ofpins 49 and 5% provided by the ears thereof which are loosely receivedin enlarged slots or openings 51 and 52 provided by the governor plates.With this arrangement, rotation of the rotor 30 tends to pivot theweights 47 and 48 outwardly about the respective axes 43 and 44 of thegovernor leaves, whereupon the opposite ends of those leaves whichprovide the enlarged slots 51 and 52 swing inwardly thereby pulling thepins 49 and 5t) inwardly and, of course, the shorting shoes 34 and 35affixed thereto. At some predetermined speed. the governor weights 47and 48 will move outwardly to an extent sufficient to withdraw theshorting shoes from ring 32, as shown in Fig. 3. On the other hand, whenthe rotational speed of the rotor falls below such predetermined speed,the weights will move inwardly and the shorting shoes will be forcedinto engagement with ring 32.

The predetermined speed at which the resistance of the rotor is alteredby removal of the shorting shoes 34 and 35 from engagement with the ring32 is determined by the tension of the springs 53 and 54. One end ofeach spring is anchored to a stake 55 slidably carried by a lug or ear56 provided by the leaf 45. That stake has a threaded end that receivesa nut 57 thereon. The opposite end of each spring is secured to a stake58 slidably carried by a lug 59 provided by the leaf 46. The outer endof the stake 58 is also threaded so as to receive a nut 60. The tensionin the springs 53 and 54 is controlled by the position of the nuts 57and 60 along the respective stakes 55 and 58. Tension is increased bythreading the nuts farther onto the stakes, and is decreased by movingthe nuts in the opposite direction. One adjusting means might besufficient, but for manufacturing simplicity it is preferable to makethe leaves 45 and 46 identical. 7

The modification illustrated in Figs. 4 and 5 comprises a rotordesignated generally with the numeral 70, and its general constructionand electrical characteristics are quite similar to those of the rotorsheretofore described.

This rotor is mounte for rotation about a shaft and is provided with aplurality of bars separated from each other by gaps. The bars the rotorshown in Figs. 4 and 5 are electrically coup y ring 2 provided with aispaces or slots 73 as before. The gaps 73 are adapted 5 to be shortedacross by indivi ual shorting shoes or memhe rotor periphery and arebers 74 which are interior of t biased outwardly into engagementtherewith by spring 75 which at one end seat thereagainst, and which attheir other end seat on a projection 76 pro ided by a mounting member'77 rigidly secured to the s i it The shorting element 74 is affixed toa pin extencing radially of the rotor 7d in alignment with slots 73,throt it an opening in the linger as. The spring 75 slidably receivesthe pin 73, and the pin in turn is slidable with respect to the finger.76 and at its inner end is provided with a lateral extension 75" thatconstrains it within a cam slot 89 provided in a cam arm til carried bya governor element 82 which is keyed on the shaft ii so as to preventrotation thereof with respect thereto but is free to move 0longitudinally therealong. The longitudinal position of the governorelement 32 determined by the governor control element which, in theillustration given, is seen to comprise an arcuate arm 34 pivotallymounted intermediate its ends on a shaft or pivot axis 85 extendingbetween the spaced legs of the bifurcated end 86 of a bracket $57 fixedto the shaft 7i. At its outer end the arm lidis equipped with a weightand at its inner end it is bifurcated to provide spaced legs 39 thatreceive an outwardly extending annular flange 9d provided by thegovernor element in operation of this form of the device, rotation ofthe rotor 743 results in the weights tending to move outwardly along anaxis normal to the rotational of the shaft '71 and through the pivotpins Such movement will cause the inner end of the arm 84- to movetoward the left as viewe Fig. 5, whereupon the governor element will bein yet toward the left with respect to the shaft i1, because of the irenation of the cam element 81, S will d inwardly and the shortingelements will tend to be moved away from ring 72. Thus at somepredetermined speed, the described movement will be sufficient toovercome the shorting across of the gaps The form of the invention shFigs. 6 and 7 again comprises a rotor for an induction motor having thesame electrical characteristics. T e rotor is dezignated generally withthe nuni F and it is supported for rotation on a shaft 1&1, is equippedwfth the usual plurality of bars separated freq each other by slots orgaps. The bars in the rot -r :cwn are electrically coupled by means avided w slots 1433 extending partwa thereacross. are adapted to beshorted across by a s or in the form of a plate h edge 1% adapted toengage thus bridge slots The plate at its inner equipped with a hub id?mounted on the shaft longitudinal movement therealong, but u is ablykeyed to the shaft so as to constrain a se relative rotational movement,

Afixed to the shaft ding inwar l axis of the shaft 3 h'xtending betweenthe spaced prongs of the bi'" sated iingers is a pivot pin that providpivotal moun l for an angular arm M2 intermedia M2 at its outer endcarries a weigh a and at it. or end is bifurcated to provide spaced legs134 that receive a collar 115 therebetween carried by t'ie hub lit? itthe governor element or shorting shoe 1%.

The arm 112 carri s an apertures ear larly, the member carries anaperture?! ea: aligned therewith. A coil spring 118 extends bettne twocars and has an tnereof booked in each of the apertures. Therefore, thespring 118 provides a biasing force that tends to draw the weight 113inwardly, whereby the shoe res is forced into engagement with the endsof the ring 1%. it is apparent that the governor structure described, ormore pa ticularly the arm weight 133, bifurcated finger 9, etc, has acounterpart diaine cally opposite thereto, since such counterpartstructure l identical, no further descri tion thereof will be set forth.

and 7, the s against the ring r relatively low. v etermined to moveshorta SllifiClBl'li amount so 3 toward the left along it-will beseparated from the ring of the rotor will t be increased to against thespeed thereof. i ployed in connection with to x axis designated torquerevolutions per minute. of identification, are des through determincc oe liter ls a. v plotted for a prevolt c for ction motor. The curve i wsthe relations of to r speed for a rotor raving a norm re curve 121 isfor a rotor ha resistance.

clear then by comparin the wh n the resistance of the r tor is high.

AS a it is apparent that speed con :01 for the motor can erT-ectuated bydecreasing the rotor resistance when tn speed thereof tends to fall off.

The curve 122 shown in broken lines corresponds to the curve 123, but itis r presentative of the rotor speedtorque relationship at a loweroperating voltage. The curves 1T3 and have the sameinterrelationship-the curve simp y representing the lower operatingvoltage for a rotor having a high resistance. it will be noted that thetorque drops off much more rapidly with a decrease in the operatingvoltage of the motor when the resistance of the rotor is high, than whenthe rotor resistance is low; and this relationship is graphicallyrepresented by the respective spacings between the curves 122i, 123 and12%, 122.

To illustrate the manner in which change of resistance can provide speedcontrol, line is provided on Fig. 8 to show the torque requirements of atypical load which might be imposed upon a motor constructed inaccordance with teachings of this invention. Ordinate y shows the speedat which the rotor of the motor assumes a high resistance under thecontrol of the governor mechanism. Ordinate y shows the speed at whichthe rotor of the motor assumes a low resistance or operates in the shortcircuited condition underthe control of the governor. The intersectionof ordinate y with line 124- establishes the a inium speed and torque asshown by abscissa x vhicn will be attained during the driving of thisparticular The interse n of ordinate y with l c 32:? estabaoscissa x anddetermines the minimum speed and torque which will occur.

Thus it is seen that the speed variation between ordinates 3 and y; ismaintained by the action of the governo mechan' i and r y be any amountdesired de vity of the governor. For exy ran e between 5 and 25 revo-.:.m;ite So long as the intersections of ordinate; y, and y with line ifall between lines 121' and the governor has complce control over thespeed of the motor driving this particular load.

The various embodiments of the invention heretofore describe in detailexemplify a system for controlling the speed of an induction motor bydecreasing rotor resistance when the speed thereof tends to fall below apredetermined value, and for increasing the resistance thereof when thespeed attains such preselected value. Thus, the induction motor is madevoltage-sensitive and voltage-insensitive as required in order tomaintain a substantially constant operating speed therefor, whereby suchmotor is relatively independent of fluctuations in the operating voltagetherefor. The control is eifectuated in the low voltage circuit of themotor whereby there is no interference with other electrical apparatusand appliances in proximity therewith-radio and television receivers,for example; and further, there is but a minimum of burning and wearingof the shorting contacts. Simply as an indication of the low voltageemployed, the potential between adjacent parts might be in the nature ofabout /6 volt.

In addition, there is no problem in the system disclosed of transferringmechanical motion from rotating to nonrotating parts since all of themechanism used to cite:- tuate the speed control is carried by therotor. Furthermore, the system and the apparatus disclosed lends itselfto adjustment whereby the motor speed may. be made substantiallysynchronous at any preselected speed, and any desired degree of controlsensitivity may be attained.

While in the foregoing specification embodiments of the invention havebeen described in considerable detail for purposes of making a completedisclosure thereof, it will be apparent to those skilled in the art thatnumerous changes may be made in those details without departing from thespirit and principles of the invention.

I claim:

1. In a speed control system for an induction motor, a rotor adapted tobe rotated and being equipped with a plurality of conductorselectrically coupled together by circular conductor means,said'conductor means having spaced areas of low and high resistance,means for shunt ing said areas of high resistance to decrease theelectrical resistance of said circular conductor means and hence saidrotor, and means for removing said shunting means when the rotationablevelocity of said rotor exceeds a predetermined value.

2. The system of claim 1 in which said last mentioned means comprisesgovernor apparatus carried by said rotor operatively responsive to thevelocity thereof.

3. In a speed control system of the character described for use ininduction motors, a rotor for such motor having a plurality of conductorbars electrically operated from each other, circular conductor means forcoupling said conductor together, said means having spaced portions ofgreater and less cross-sectional area, means for bridging the saidportions of less cross-sectional area and governor means carried by saidrotor and being connected with said last mentioned means for moving thesame from bridging to unbridging position when said rotor exceeds apredetermined velocity.

4. The control system of claim 3 in which said circular conducting meansis provided with air gaps extending partway therethrough todefine'portions of less crosssectional area, and in which said bridgingmeans comprises shorting members for bridging such gaps.

5. The speed control means of claim 3 in which said governor means isadjustable to afford selection of the predetermined rotor velocity atwhich said bridging means is shifted from one position to another.

6. In speed control apparatus for an induction motor having a rotorprovided with conductors electrically coupled together with a partiallyslotted conducting ring, conductor shoes shiftable into engagement withsaid ring over a slotted portion thereof, means for urging saidconductor shoes into engagement with said ring, and governor apparatusincluding said means and being connected with said conductor shoes forshifting the same out of contact with said ring when said rotor exceedsa predetermined rotational velocity.

7. The apparatus of claim 6 in which said governor apparatus comprises aplate carried by said rotor, a pin for each of said ring slots andconnected to said plate, the respective conductor shoes being slidablymounted thereon, and a spring for each shoe biasing the same intoengagement with said ring.

8. The apparatus of claim 7 in which said plate is adjustably rotatablewith respect to said rotor and is provided with a plurality of angularlyoriented cam slots, one for each of said pins, and in which each pin isslidably connected to said plate through a cam slot therefor, eachspring being secured to a pin at one end and bearing against theconductor shoe at its other end to urge it toward contact with saidring.

9. The apparatus of claim 6 in which said governor apparatus comprises apin for each of said conducting shoes to slidably receive the samethereon, a spring for each of said conducting shoes urging the same intoen gagement with said ring, a governor control member longitudinallymovable with respect to the rotational axis of said rotor and having aninclined cam slot for each of said pins whereby each pin and theconducting shoe mounted thereon is moved toward and away from said ringupon lonigtudinal movement of said governor control member, andfiyweights carried by said rotor and being operatively connected withsaid governor member for shifting the same longitudinally in accordancewith the rotational velocity of said rotor.

10. The apparatus of claim 6 in which said conducting shoes areelongated arcuately for bridging a plurality of slots in said ring, inwhich weight-equipped governor control elements are pivotally connectedintermediate the ends thereof to said rotor and are pivotally connectedat an end thereof to said conductor shoes, and in which spring means areemployed to normally bias said governor control elements into a positionin which said conducting shoes are in contact with said ring.

11. The apparatus of claim 6 in which a unitary conducting shoe isprovided for engagement with the end of said ring and is shiftablelongitudinally with respect to the rotational aXis of said rotor forselective engagement with said ring in which governor weights arecarried by said rotor and are coupled with said unitary conducting shoefor establishing the position thereof, and in which spring means areprovided for urging said governor weights into a position in which saidunitary conducting shoe is in engagement with said ring.

References Cited in the file of this patent UNITED STATES PATENTS1,002,837 Gray Sept. 12, 1911 1,518,952 Arutunolf Dec. 9, 1924 2,748,333Lee May 29, 1956

